Electrical capacitors with ester impregnants

ABSTRACT

Esters are used as impregnants in AC and energy storage capacitors, wherein the esters have alkyl groups substituted around the carbonyl carbon atom to effectively prevent hydrolytic attack thereof by water or hydroxide ions. Appropriate substitution in both the acid and the alcohol moieties insures a hydrolytic stability for the esters produced thereby.

[ June 19, 1973 United States Patent 1 Ross et al.

[56] References Cited UNITED STATES PATENTS ELECTRICAL CAPACITORS WITH ESTER IMPREGNANTS [75] Inventors: Sidney D. Ross, Williamstown;

3,363,156 1/1968 Cox................. ...................317/259 Manuel Finkelstein, North Adams, both of Mass.

Primary ExaminerE. A. Goldberg Att0rneyVincent H. Sweeney and Connolly and Hutz v [73] Assignee: Sprague Electric Company, North Adams, Mass.

Nov. 4, 1971 [21] Appl. No.: 195,856

[57] ABSTRACT Esters are used as impregnants in AC and energy storage capacitors, wherein the esters have alkyl groups substituted around the carbonyl carbon atom to effec- [22] Filed:

[ 52] U S Cl tively prevent hydrolytic attack thereof by water or hy- 51] I t Cl 317/259 252/ 25 9 2: droxide ions. Appropriate substitution in both the acid and the alcohol moieties insures a hydrolytic stability 1 0 can for the esters produced thereby.

10 Claims, 1 Drawing Figure Patented Jun 19, 1973 3,740,625

ELECTRICAL CAPACITORS WITH ESTER IMPREGNANTS BACKGROUND OF THE INVENTION This invention relates to ester impregnants for electrical capacitors, and more particularly to esters having alkyl groups substituted therein so as to substantially surround the carbonyl carbon atom.

A suitable dielectric oil for AC and energy storage capacitors must have the following physical characteristics: l Freezing point and pour point below 40C;2) Negligible vapor pressure at 125C and 100 microns; 3) Flash point greater than 350F; and 4) A viscosity of 1,000 centipoises or less at 100C. The required electrical parameters for same are: l) a dielectric constant greater than 2.4 but smaller than 2) a volume resistivity in excess of 10 ohm-cm; and 3) a power factor of less than 0.1 percent and preferably less than 0.05 percent.

Many esters are capable of meeting the foregoing requirements, but have been used infrequently because of chemical and economic reasons. The chemical reason is that most esters lack sufficient hydrolytic stability. In the presence of water or hydroxide ion, the ester is in equilibrium with the components from which it is made namely, an acid and an alcohol. The hydrolysis generates an acid which decreases the resistivity of the dielectric oil and promotes degradation of the unit.

The hydrolysis of an ester involves attack by water or hydroxide ion at the carbonyl carbon atom. The reaction can be both acid and/or base catalyzed, but a typical dielectric fluid would be essentially neutral, and the hydrolysis of concern would involve attack by a neutral water molecule. But such attack can be significant, particularly at the elevated temperature at which capacitors frequently must operate. Recently, polyolefin film capacitors, for example, polypropylene, have been reported to be capable of being fully impregnated with a dielectric liquid of the type comprising a halogenated aromatic compound having from one to five chlorine substituents and from one to three aryl groups. It has been determined, however, that it is only with extreme difficulty that this type of unit can be impregnated, even when employing a paper dielectric film as a wicking aid in conjunction with the'polypropylene 'film.

tween the electrodes. The capacitor is fully impregnated with at least a major proportion of appropriately substituted esters. The esters are virtually insensitive to hydrolytic attack because of the appropriate substitution in both the acid and alcohol moieties used to prepare these esters.

The attack by water can be virtually eliminated by surrounding the carbonyl carbon atom by alkyl groups. The introduction of two or even one alkyl substitutents at the 2 position of the acid or the alcohol may give an ester sufficient hydrolytic stability to be useful as a dielectric fluid.

Branched alkyl groupings used therein mesh and fit well with the highly branched molecular structure of the polypropylene film that has recently been used in some capacitor applications.

BRIEF DESCRIPTION OF THE DRAWING The drawing illustrates a'capacitance section 10 of the type with which the present invention is concerned. This section will be positioned within a capacitor container or housing means (not shown) and later impregnated with the fluid dielectric of the present invention.

Section 10 comprises a convolutely wound section having metal electrodes 11 and 12 separated by dielectric spacers l3 and 14, such as polypropylene film or Kraft paper. The capacitance section has electrode tabs 15 and 16 which are in contact with electrodes 11 and 12 and which will thereafter make contact to the underside of a conventional capacitor can cover assembly containing tab terminals.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Esters can be used advantageously as dielectric fluids when alkyl groups having straight or branched chains are substituted at the carbon atoms that are a" to the ester'carbonyl and/or B to the ester ether linkage, that is, when the ester is made from Z-substituted acids and 2-substituted alcohols. This substitution effectively renders the ester impervious to hydrolytic attack by water or hydroxide ion, and makes the ester more compatible with a polyolefin film such as polypropylene.

The hydrolysis of an ester involves attack by water or hydroxide ion at the carbonyl carbon atom. The reac5 tion can be both acid and base catalyzed, but a typical In view of this difficulty, it has been found necessary,

for the impregnation of commercial units, to employ with the polypropylene film, a porous cellulosic dielectric, e.g. Kraft paper, and also to employ special heat cycling procedures, after initial impregnation of the unit, in order to achieve maximum impregnation of the device. Such meansobviously increase the cost of the unit.

Accordingly, it is an object of the present invention to provide ester impregnants for electrical capacitors that are virtually insensitive to hydrolytic attack.

It is another object of this invention to provide ester impregnants for use in polypropylene film capacitors that mesh and fit well with the highly branched molecular structure of the polypropylene film.

SUMMARY OF THE INVENTION housing means. The capacitance section comprises at least a pair of electrodes and a dielectric spacer bedielectric fluid would be essentially neutral, and the hydrolysis of concern would involve attack by a neutral water molecule. Where two alkyl groups have been introduced in the 2 position of the acid moiety, and two alkyl substituents have been introduced in the ,2 position" of the alcohol moiety, the ease of hydrolysis is diminished by as much as four orders of magnitude.

While such an extreme degree of substitution is desirable, since it affords a maximum margin of safety, it is not essential. In specific cases, the introduction of two or even one alkyl substituents at the 2 position" of the acid or the 2 position" of the alcohol may result in an ester having sufficient hydrolytic stability to be useful vent such as benezene in the presence of a base such as pyridine to produce the ester.

Advantageously, the esters used should have a dielectric constant greater than 2.4 but smaller than 10. If the esters dielectric constant is in excess of 10, this will frequently result in the material having ionic contaminants present that are very difficult, if not presently impossible, to remove. Esters having an appropriate dielectric constant are easily obtained. Some of these include dibutyl phthalate, a C ester that has a dielectric constant of 6.43 to 30C; amyl benzoate, a C,, ester, has a dielectric constant of 5.00 at 20C; ethyl dodecanoate, a C ester, has a dielectric constant of 3.44 at 20C; and dioctyl phthalate, a C ester, has a dielectric constant of 5.1 at 25C. Since most pure hydrocarbons have a dielectric constant close to two, the origin of the higher dielectric constants is the dipole moment of the ester grouping. Therefore, esters have significantly more or significantly less alkyl substitution than those cited above would still have dielectric constants of the appropriate magnitude.

The flash points of esters can be increased by increasing the molecular weight and thus decreasing the'volatility, and esters with flash points in excess of 450F are plentiful. As a class, the esters have low viscosities, and esters having'flash points in excess of 500F frequently have viscosities of less than centipoises at 210F.

it is more advantageous to impregnate the capacitance section with a liquid dielectric and one that does not set upon standing, as such an arrangement produces' air spaces or air gaps therein that inhibit complete impregnation and result in failure of the capacitor under voltage because of sparking and corona. There fore, when using esters herein that are solids, it is preferred that blends are prepared with other liquid esters of this invention so as to insure that the ester to be used as the impregnant is in liquid form.

we have found the following classes of esters to be suitable for use in dielectricfluids:

wherein R, is a C,--C, alkyl group; R is a member selected from CH, and H; R, is CH, or H; R, is a member wherein R, is a C,C, alkyl group; R is a CH or C H and R, is Cl-i or H.

Preferred embodiments are those in which R, is C H', and R, is H; and those in which R R CH,,.

[its (ll it its 1 wherein R, is a C,C, alkyl group; R, is a CH, or H;

wherein R, is a C,C, alkyl group; R is CH, or H; R, is CH, or C H,; and R, is CH, or C,H

(1m 0 I ll ammo wherein R, is a C,-C, alkyl group.

It should be noted that the starting materials for this ester, hydroxypivalyl hydroxypivalate,

l 011, cm

and the neo-acids,

(Illa hol stocks readily available from the chemical and petrochemieal industry for the preparation of the desired 5 esters. This, of course, is more economical than using the pure compounds.

Some available acids that can advantageously be used as starting materials for the purposes of this invention include: the neo-acids, 2-methylpentanoic acid; 2-ethylbutyric acid; 2ethylhexanoic acid; phthalic acid; and trimellitic acid'(available as an anhydride). Available alcohols include: 2,2,4-trimethylpentanol; 2- methylpentanol; 2-ethylhexanol; Z-ethylisohexanol; neopentylglycol; 2-ethylbutanol; pentaerythritol; trimethylolethane; trimethylolpropane; and hydroxypivalyl hydroxypivalate.

These appropriately substituted starting materials are readily available and/or can easily be made. Amongst the chemical processes capable of making these are the 0x0 process for the preparation of branched alcom' mo' hols, and the catalyzed addition of carbon monoxide to olefins to form neo-acids.

The freezing point of these esters should be below 40C, while the vapor pressure should be negligible at 5 100 microns and 125C for paper capacitors and 100 microns and 100C for plastic film capacitors. The viscosity must not exceed 1,000 centipoises at the impregnation temperature (125C for papers capacitors and 100C or less for film capacitors). The power factor of these materials should be less than 0.1 percent for paper and less than 0.05 percent for film capacitors. And, a flash point in excessof 350F is needed.

Various compounds derived from the broad examples of available esters previously listed, and some of 15 their respective properties are as follows:

The esters listed herein can advantageously be used as dielectric impregnants. Various mixtures or blends thereof have also been found to meet the requirements for dielectric impregnants for paper and/or film capacitors. The capacitance section of a capacitor is to be fully impregnated'therewith.

The polymer films contemplated by the present invention include polypropylene, polyethylene, polystyrene, etc. It is preferred to employ isotactic polypropylene of fairly high purity. This type of polypropylene is presently commercially available from several different sources. For some purposes it is advisable to employ in conjunction with the polymer filma porous cellulosic type spacer to assist in impregnation and to act as a wick for the liquid impregnant. A commonly employed material for this purpose is Kraft paper. It is also contemplated that one or both of the capacitor electrodes may be in the" form of a thin metallization on the surface of the polymer film. An example of this form is electrodes may be in the form of a thin metallization on each surface of a porous paper such that the rnetallization on one surface thereof contacts the metallization on the other surface,the electrodes being separated by a plastic film dielectric. Because these esters are so stable, they should not produce any acid, and therefore are advantageously compatible with metallized electrodes, unlike the chlorinated hydrocarbon impregnan used in the prior art. 1

Since it is obvious that many changes and modifications can be made in the above-described details without departing from the nature and spirit of the invention, it is to be. understood that the invention is not limited to. said details except as set forth in the appended claims.

What is claimed is: a

' 1. An electrical capacitor comprising a capacitor housing, a capacitance section within said housing, said section having at least a pair of electrodes and a dielectric spacer between said electrodes, said section being impregnated with a liquid dielectric comprising at least one ester having alkyl groups substituted at'a position on said at least one ester selected from the group consisting ofoz to the ester carbonyl and B to the ester ether linkage, said alkyl groups being a member selected from H, CH and C l-l I 1 2. The capacitor of claim 1 wherein said liquid dielectric is at least one alkylsubstituted ester of the formula wherein R, is a C -C alkyl group; R is a member selected from CH and H; R is a member selected .from

. CH and H; R, is a member selected from CH C H and H; R is a member selected from CH C l-l and H; and at least one of the groups from R to R;, is not H.

3. The capacitor of claim I wherein said liquid dielectric is at least one alkyl substituted ester of the formula aluminized polypropylene. And further, the. capacitor 4. The capacitor of claim 1 wherein said liquid dielectric is at least one alkyl substituted ester of the formula I wherein R1 is a C1-C1: alkyl group; R2 is a member selected from CH and C H and R is a member selected from CH and H. I

5. The capacitorof claim 1 wherein said liquiddielectric is at least one alkyl substituted ester of the forwherein R is a C,C alkyl group; R is a member selected from CH or H; and R isa member selected from CH and C l-l 6. The capacitor of claim 1 wherein said liquid dielectric is at least one alkyl substituted ester of the formula electric is at least one alkyl substituted ester of the formula (Inn cm 0 cm- 0 tin ltr-(I llz(,c O-CH2(ECHzO-C(%-ClIzOI-Cll2-R| I CH3 CH3 CH3 CH3 wherein R is a C, -C alkyl group. spacer is a polyolefin film and wherein at least one of v 8. The capacitor of claim 1 wherein said electrodes said electrodes is in the form of a metallized layer deare in the form of a thin 'metallization on each surface posited on a surface of said polyolefin film. of a porous paper. 10. The capacitor of claim 9 wherein said metallized 9. The capacitor of claim 1 wherein said dielectric l0 layer is aluminum; a 

2. The capacitor of claim 1 wherein said liquid dielectric is at least one alkyl substituted ester of the formula wherein R1 is a C1-C12 alkyl group; R2 is a member selected from CH3 and H; R3 is a member selected from CH3 and H; R4 is a member selected from CH3, C2H5 and H; R5 is a member selected from CH3, C2H5 and H; and at least one of the groups from R2 to R5 is not H.
 3. The capacitor of claim 1 wherein said liquid dielectric is at least one alkyl substituted ester of the formula
 4. The capacitor of claim 1 wherein said liquid dielectric is at least one alkyl substituted ester of the formula
 5. The capacitor of claim 1 wherein said liquid dielectric is at least one alkyl substituted ester of the formula wherein R1 is a C1-C12 alkyl group; R2 is a member selected from CH3 or H; and R3 is a member selected from CH3 and C2H5.
 6. The capacitor of claim 1 wherein said liquid dielectric is at least one alkyl substituted ester of the formula wherein R1 is a C1-C12 alkyl group; R2 is a member selected from CH3 and H; R3 is a member selected from CH3 and C2H5; and R4 is a member selected from the group consisting of CH3 and C2H5.
 7. The capacitor of claim 1 wherein said liquid dielectric is at least one alkyl substituted ester of the formula wherein R1 is a C1-C12 alkyl group.
 8. The capacitor of claim 1 wherein said electrodes are in the form of a thin metallization on each surface of a porous paper.
 9. The capacitor of claim 1 wherein said dielectric spacer is a polyolefin film and wherein at least one of said electrodes is in the form of a metallized layer deposited on a surface of said polyolefin film.
 10. The capacitor of claim 9 wherein said metallized layer is aluminum. 